There're 3 lines. A good one to take would be the
Powell-Mason line since it takes you from Powell BART station
to Fisherman's Wharf, another
San Francisco tourist attraction.
Unless you buy a pass, the cost is US$5 per ride (at the time of writing).
If you have a strong enough grip, ride it standing on the sidebar would be more exciting,
especially going downhill. Just don't say 'Look, mom, no hands!'